1. Field of the Invention
The invention relates to an oil drain passage structure for guiding, to an oil pan, lubrication oil in a space surrounded by a side wall of an engine block on the side on which a timing chain is arranged and by an inner wall of a chain cover that covers the timing chain.
2. Description of the Related Art
The rotation of a crankshaft of an internal combustion engine is transmitted to a camshaft by means of a timing chain. This timing chain is typically covered by a chain cover. Lubrication oil for lubricating the timing chain and lubrication oil discharged from a hydraulic tensioner for adjusting the tension of the timing chain are dispersed within a space surrounded by a side wall of an engine block on the side on which the timing chain is arranged and an inner wall of the chain cover, namely on the inner wall of the chain cover. In addition, lubrication oil that has passed through a cylinder head is discharged to the inside of the chain cover.
When lubrication oil within this chain cover contacts the timing chain while it is rotating or accumulates on a wall surface of the chain cover, air bubbles easily enter the lubrication oil as a result of the lubrication oil being agitated, thereby resulting in the risk of, for example, a decrease in hydraulic pressure of the lubrication system or an inadequate supply of lubrication oil.
For example, Japanese Patent Application Publication No. 8-200089 (JP-A-8-200089) describes that a rib formed on an inner wall of a chain cover is used as a partition, and an oil drain passage is formed by providing the partition extending towards the bottom of an engine block, thereby enabling lubrication oil within the chain cover to be guided towards an oil pan via this oil drain passage.
In the case of forming an oil drain passage in the manner described above, lubrication oil flowing over the inside of the partition that forms the oil drain passage is suitably returned to the oil pan. However, lubrication oil flowing over the outside of the partition that forms the oil drain passage is not returned to the oil pan, but rather ends up accumulating on the wall surface of the chain cover, thereby resulting in susceptibility to the formation of air bubbles.